1. Field of the Invention
The invention concerns a filter element, particularly one intended for use in removing contaminating particles from a fluid. The invention also concerns filter elements which can be put to such diverse uses as the removal of water from oil, the catalysis of a liquid, the removal of color from a liquid, or the chlorination of water
2. Description of the Related Art
Filter elements often employ a plurality of layers of filtering material, thus prolonging their useful life by collecting only a portion of contaminant at each layer. If the layers are identical, most of the contaminant will be collected at the upstream layer. To counteract this, some filter elements incorporate a gradient density such as is provided by a stack of filtering fabrics of progressively reduced pore size so that larger particles are collected by upstream layers while smaller particle are collected by downstream layers. See, for example, Thomas U.S. Pat. No. 3,003,643. When the contaminant has a broad size distribution, such a filter element gives good depth loading performance and hence has a long useful life. However, when the contaminant size distribution is narrow, it is primarily collected within a thin zone so that the filter element becomes clogged or otherwise ineffective even though most the depth of the filter element remains relatively free from contaminant.
Japanese patent Application No. JA57-11264, which was laid open July 30, 1983 (Publication No. JA58-128121), shows a filter element formed by winding a long strip of filter paper on itself. The filter paper is formed with a large number of small holes which are arranged so that the holes in any convolution do not coincide with the holes of adjacent convolutions. The application says that part of the oil being filtered passes through the holes, and the other part passes through the portion of the filter paper between the holes, whereby less fluid loss may be achieved.
Gimbel, U.S. Pat. No. 4,629,569 shows a device called a "depth filter" to be substituted for a gravity-driven granular bed such as is commonly used for the treatment of water. Gimbel's depth filter consists of "permeable, thin membrane layers which are disposed at small mutual spacings of for example approximately 0.5-10 mm, preferably 1-5 mm. In order to insure depth action of the filter larger circular or disk-shaped holes, so called macroholes of, for example 0.1 to 5 mm are provided in these membrane layers at predetermined spacings; through these holes the major portion of the liquid throughput continues to flow largely unimpeded--even after a long period of filter operation . . . Preferably, the macroholes are made smaller, continuously or step-wise, in the individual layers in the principal flow direction" (col. 2, lines 1-37). In order to maintain the aforementioned spacing of "large-area membrane layers, it may be necessary to insure the horizontal attitude of the membrane layers by the installation of suitable support elements between the individual layers. As a matter of principle however it is also possible to provide the membrane filter surfaces in suitable frames which then are combined, side by side, to form larger filter surfaces in order to avoid the sagging of the membrane filter layers" (col. 5, lines 41-49).